Setting up a back end is easy. What languages do you know? I can help you on the way if you want to chat with me. I've done firebase, Amazon Web Services, and Azure. A good place to start is to look up a tutorial about Amazon Web Services. This looks pretty nice.
Firebase makes web development super easy, and there's been success stories of people who have thousands of users. I don't know how they scale, but it's weird to hear that they're not good with scaling.